1. Industrial Utilization Field
The present invention relates to an apparatus for detecting the deterioration of an oxygen sensor which is disposed in an exhaust system of an internal combustion engine in an air-fuel ratio controller of the engine and which outputs signals according to oxygen concentrations in exhaust gases.
2. Prior Art
Recently, as means for diminishing harmful exhaust gases from vehicles and improving fuel economy and drivability, there has been proposed a feedback type air-fuel ratio controller which controls the air-fuel ratio on the basis of information on exhaust gas components from an internal combustion engine such as a vehicular engine.
In the air-fuel ratio controller of the type just mentioned above, in the event of trouble of an exhaust gas sensor for detecting the concentration of an exhaust gas component or of any other portion, there is not performed a normal control and it is likely that the air-fuel mixture will become overrich or overlean with respect to an appropriate value. Once the air-fuel mixture becomes lean, the operating characteristic and stability of the engine are deteriorated, and an overrich condition of the mixture gives rise to problems such as, for example, an increase in the proportion of harmful components contained in exhaust gases. Therefore it is necessary to promptly detect an abnormal condition of the air-fuel ratio controller and take appropriate measures.
In such air-fuel ratio controlling method, an abnormal condition of a component in exhaust gases or troubles in the control system occur in the case where a proper control cannot be made due to, for example, failure or deterioration of a sensor used, e.g. oxygen sensor (hereinafter referred to simply as "O.sub.2 sensor"), itself. Particularly, since the O.sub.2 sensor is in many cases disposed near an engines, it is directly influenced by high temperature and pressure and vibrations, so is apt to be deteriorated. On the other hand, since the O.sub.2 sensor detects exhaust gases just after discharge from the engine, the composition of components is extremely unstable and thus the sensor is apt to be influenced by the cycle of the engine, so it is desired for the O.sub.2 sensor to always have an extremely high detection accuracy.
Therefore, when the detection accuracy of the O.sub.2 sensor is deteriorated for some reason or other, it is necessary to immediately detect this condition, clear up the cause and take appropriate measures, for example, replacement of the sensor with a new one. However, means capable of detecting a deteriorated state of the O.sub.2 sensor easily and exactly has heretofore been not available.
As a method for detecting the deterioration of the O.sub.2 sensor there is known the method disclosed in Japanese Patent Laid Open No. 81824/78. According to this method, an O.sub.2 sensor is disposed between an exhaust port of an engine and a catalytic converter, and when the frequency of air-fuel ratio feedback (F/B) carried out using an output of the O.sub.2 sensor has become lower than a predetermined value, it is judged that the sensor is deteriorated, by utilizing the phenomenon that said F/B frequency becomes lower with deterioration of response characteristic.
However, there are two kinds of deterioration patterns of the O.sub.2 sensor, in one of which the rising of Z characteristic of the sensor deviates from a theoretical air-fuel ratio, that is, the air-fuel ratio control center deviates, as shown in FIG. 10(a), while in the other pattern, the response characteristic of the O.sub.2 sensor is deteriorated, as shown in FIG. 10(b). In the type (a) deterioration, the F/B frequency itself is almost the same as in the normal state. On the other hand, once the type (b) deterioration occurs, the detection itself of rich/lean reversal is delayed and the air-fuel ratio is controlled in the reverse direction after actual occurrence of an overrich or overlean condition, thus resulting in that the F/B period becomes very long, that is, the frequency becomes smaller.
According to the above prior art, since the deterioration of an O.sub.2 sensor is detected on the basis of frequency, it is impossible to detect the (a) type deterioration although the (b) type deterioration can be detected. However, since an actual deterioration of emission occurs in a combination of the foregoing two patterns of deteriorations, it is necessary to detect both (a) and (b) types of deteriorations accurately at a time.